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Helping Kids Understand Sir Isaac Newton’s Three Laws of Motion

Your son or daughter has science questions about Sir Isaac Newton’s three laws of motion. How can you begin to guide your child’s understanding of these concepts? Without thinking about them, we use Sir Isaac Newton’s three laws of motion every day. Newton’s first law explains why it is harder to stop a moving car than a roller skate. Newton’s second law algebraically relates the force on an object, its mass, and its acceleration. Newton’s third law concerns how forces act upon objects. By relating every day experiences, you can help your child understand Sir Isaac Newton’s three laws of motion.

Newton’s first law of motion is also known as the law of inertia. The term, inertia, derives from the Greek, inert, or not moving. Newton’s first law states that any object will remain stationary or will continue to move in a straight line unless it is acted upon by an external, unbalanced force. A force is a push or pull on an object. Inertia is a measure of the mass of an object. An automobile has more inertia than a roller skate. While you are traveling in a moving car, you are moving in the same direction and with the same speed as the car. If the car suddenly comes to a stop, you will still be moving in the original direction, through the windshield if you do not use a seatbelt or airbag. The seatbelt keeps you in one position relative to the car’s motion, keeping your body against the seat. Inertia also explains why you lean towards the opposite direction as the car moves around a steep curve. If the car turns right, you lean towards the left; if the car turns left, you lean towards the right. Again, your body continues to move in a straight line during the turn, as it did before the turn.

Any time you want to change the speed or direction of an object, you need to use the appropriate force. Newton’s second law of motion relates the concepts of mass, force, and acceleration. In science, acceleration is the change in speed or direction of a moving object. Force on an object is equal to its mass multiplied by its acceleration. The strength of the force on an object depends upon the object’s mass, or how much material it contains, and how fast its speed is changing, or its acceleration. An automobile hitting a wall at the same speed as a roller skate would have more force, since the car has more mass. A unit of measurement for force is the Newton, abbreviated N, named after Sir Isaac Newton. One Newton, or one N, is the force needed to move a mass of one kilogram one meter per second in a second. Or algebraically,

1 N =

1 kg

ms2

A Newton of force is a small amount. A person weighing 110 pounds exerts a force of 50 Newtons on Earth.

Newton’s third law of motion is more commonly called action reaction. For every action in one direction, there is an equal and opposite reaction in the opposite direction; even if the object does not move. Forces always act in pairs, even if the object remains still. While sitting in a chair, you provide a force on the chair acting down towards the floor. At the same time, the chair provides an equal and opposite upward force on you. If this were not the case, you would be sitting upon the floor instead. While you walk, for each step that you take your foot pushes against the floor. As you push, or provide a force, against the floor, the floor also pushes against your foot, propelling you forward. If you try to walk across sheer ice, you must adjust your steps, since the ice does not provide the same force as the floor.

By using every day examples, you can help your children understand Sir Isaac Newton’s three laws of motion. The law of inertia, or Newton’s first law of motion, describes how a stationary object begins to move or how the motion of an object changes. Newton’s second law of motion algebraically relates an object’s mass and acceleration to the amount of force involved to cause motion. Finally, Newton’s third law of motion involves the fact that forces on an object always act in opposing pairs, whether or not the forces cause motion.

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